1. Field of the Invention
The present invention relates to a process for preparing hydroxyalkyl (meth)acrylates by reacting epoxides and carboxylic acids in the presence of Lewis acid catalysts.
2. Description of Related Art
In the context of the present invention, (meth)acrylate signifies the esters of acrylic acid, methacrylic acid or mixtures thereof. Hydroxyalkyl (meth)acrylates are used, inter alia, for reaction with isocyanate-containing compounds in order to prepare urethane (meth)acrylates, unsaturated polyurethane dispersions and dual cure hardeners for two-component or multi-component coating systems. The previously mentioned classes of compounds are used in particular as constituents of coating agents which cure by radical polymerization. This process can be triggered by actinic radiation. Curing via a combination of two reaction mechanisms is called dual cure by a person skilled in the art.
In order to obtain particularly highly cross-linked coatings, it is expedient to use compounds with high functionality, in particular with a large number of radically polymerizable double bonds, which can advantageously be produced by the use of hydroxyalkyl (meth)acrylates with a high functionality of acrylate and/or methacrylate groups. In order to facilitate the production of a targeted molecular structure, and thus, inter alia, a low viscosity, it is also expedient to use hydroxyalkyl (meth)acrylates in which the hydroxy functionality is distributed as narrowly as possible. Therefore, the preparation of hydroxyalkyl (meth)acrylates by the esterification of polyols such as trimethylolpropane or pentaerythritol with (meth)acrylic acid by a process which proceeds randomly and produces a wide distribution of hydroxy functionality, is disadvantageous.
Also, secondary products with high molecular weights are often formed. They have been identified as addition products of hydroxyl groups to the C—C double bonds of acrylates e.g. in Analytical Sciences, November 2001, vol. 17, p. 1295-1299. DE-A 19 860 041 discloses 3-acryloyloxy-2-hydroxypropyl methacrylate as a possible compound for reaction with polyisocyanates in order to produce dual cure hardeners. Details relating to the preparation of 3-acryloyloxy-2-hydroxypropyl methacrylate, however, are not described.
The preparation of 3-acryloyloxy-2-hydroxypropyl methacrylate by the reaction of glycidyl methacrylate with acrylic acid, both commercially available in high purity, and using a suitable catalyst, is described in the literature. Data on the purification/purity of the product are not given. Thus, EP-A 0 900 778 describes the reaction of excess acrylic acid in esterification reactions with glycidyl methacrylate, catalyzed by benzyltriethylammonium chloride.
3-acryloyloxy-2-hydroxypropyl methacrylate is also obtainable commercially in the fine chemical trade (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany), but the purity, determined by gel permeation analysis, is less than 60 wt. %; in particular, undesired high molecular weight fractions are found. The method of preparation is not disclosed.
A wide range of different catalysts for the reaction of glycidyl compounds with carboxylic acids is known from coating technology. This reaction is often used, for example, as a cross-linking reaction for curing lacquers. In addition to ammonium and phosphonium salts or amines and phosphines, certain metal compounds have also been described.
Certain properties of boron-containing Lewis acids are investigated in Polymer 1996, 37(20), 4629-4631. Their use as catalysts for epoxide-acid cross-linking reactions is mentioned. However, for reactions in the presence of (meth)acrylates, only catalysts which are catalytically active at low temperatures are suitable because otherwise there is a risk of the unwanted polymerization of the (meth)acrylates. Furthermore, greater selectivity of the catalysts is required for chemical synthesis as compared with a cross-linking reaction. Secondary reactions such as the Michael-analogous additions of hydroxyl or carboxyl groups to (meth)acrylates or hydroxyl-epoxide reactions lead to undesired secondary products because they have high molecular weights.
3-methacryloyloxy-2-hydroxypropyl methacrylate is sold with a purity greater than 85%, following purification by distillation, by Röhm GmbH, Darmstadt, Germany. The stability of the product is low and it has to be stored cold, which makes use on a commercially viable industrial scale difficult.
It is an object of the present invention to provide a process with which, for example, 3-acryloyloxy-2-hydroxypropyl methacrylate can be prepared, simply and at low temperatures, in high purity and without a costly purification process, i.e, without a concentration of high molecular weight or highly hydroxylated fractions.
It was found that the reaction of, for example, glycidyl methacrylate and acrylic acid, in bulk, proceeds rapidly and completely at temperatures of 80° C. by using catalysis with certain weak Lewis acid borane compounds, such as trisdimethylaminoborane.